Some of hybrid hydraulic excavators, an example of hybrid work machines, operate as follows. During work with a heavy load, such as at excavating, the excavators use both an engine and an electric motor (an assist motor) to drive a hydraulic pump, the electric motor being driven by discharge of a secondary battery and thus assisting the engine. During deceleration of an upper swing structure or boom lowering, the excavators utilize the mechanical energy coming from this operation to allow a generator motor to generate electricity. Such electric energy is subsequently charged in the secondary battery.
The charge and discharge of the secondary battery are repeated as described above during the hybrid hydraulic excavators operating. The energy from the recovered mechanical energy as above alone, however, does not easily cover all the energy discharged by the assist motor during the operation of the excavators. The state of charge (SOC) of the battery consequently decreases as the operating time passes, which can make the excavators highly likely to fail to carry out operation with a heavy load
Extra power that an engine can have during no operation or operation with a relatively light load is usually used to generate electricity with the use of a generator, such as an assist motor, to maintain the SOC of the secondary battery. This electric energy is then used to charge the secondary battery. The charge through the recovery of the mechanical energy utilizes particular motions of the operation, and so the timing of the charge cannot be selected. Yet the battery still can be charged through the electricity generation of the assist motor with the engine whenever the engine has sufficient extra power.
In this regard, JP-2011-220068-A proposes the following method. During operation, a secondary battery is quickly charged through electricity generation of an assist motor as much as possible to prevent lower battery SOC and thus maintain operability. During standby, i.e., during no operation, the amount of charge is reduced to decrease damage to the battery. Shorter lifetime of the battery is prevented accordingly.